Fast photodetectors with large saturation photocurrent are key components in high-bit-rate fiber networks and photonic microwave applications incorporating optical pre-amplification. For high-speed operation, the photodetector has to be designed for low capacitance and small carrier transit times. These considerations lead to a reduced size of the photodiode (PD), which, however, results in less high-power handling capability and a lower saturation photocurrent. One way to overcome this trade-off between speed and saturation photocurrent is to distribute symmetrically the optical signal to several photodiodes and combine their photocurrents by means of a transmission line [1,2]. Now, due to the uniform optical power distribution, the unsaturated output photocurrent scales directly with the number of photodiodes. By embedding the discrete PDs within a transmission line, a traveling wave photodetector (TWPD) can be formed of which characteristic impedance can be matched to that of the external microwave circuit and a phase match between the propagating optical and electrical signals can be achieved [3]. Since the frequency response is not limited by the overall RC time constant the bandwidth of the single photodiode can be retained, to a large extent, within the Bragg limit.The studied traveling wave photodetector chip comprises a mode field converter for effective fiber-chip coupling [4] and a 1 x 4 multi-mode interference (MMI) power splitter (width: 43 ptm, length: 1038 ptm) of which output waveguides feed four p-i-n PDs, each with an active area of 4 x 7 ptm2. The InGaAs/InGaAsP heterostructure PDs with an intrinsic InGaAs absorption layer thickness of 200 nm were optimized to provide high responsivity [5]. A coplanar waveguide (CPW) transmission line connects the PDs in parallel and collects the electrical output signal.By choosing a CPW with an impedance of 85 Q and a spacing between adjacent PDs of d= 90 pm, an impedance match of the TWPD to the 50 Q-environment as well as a phase match were calculated. The electrical Bragg frequency was determined to be >200 GHz, which is sufficiently high to provide a smooth frequency response up to more than 100 GHz. In order to eliminate electrical reflections at the input of the transmission line a matching resistor (R50 in Figs. 1 and 2) was integrated at the expense of half of the radio frequency (RF) photocurrent being lost. Details on the epitaxial layer stack and the fabrication process can be found in [5]. Using a cleaved fiber for the input, a responsivity of 0.24 A/W with a polarization dependent loss of only 0.2 dB was measured at 1.55 ptm wavelength. In order to determine the characteristic impedance and the electrical phase velocity the four S-parameters were measured using a network analyzer and contacting both ends of the CPW within a TWPD without R50. At a bias voltage Vbias -3.5 V, an impedance match of >84 00 up to 50 GHz is found. The increase in the characteristic impedance with increasing reverse bias can be attributed to the decreasing p-n junction c...